In Celtic Shamanism, there are three Worlds: Gwynvyd (The Upperworld), Abred (This World, or the MiddleWorld), and Annwn (the Underworld). The UpperWorld consists of the Celestial/Higher aspects of the Gods and Goddesses: Gods and Godesses of Light, the Sun, the Moon, the Elements, etc. Mythical Creatures such as Faeries exist here as well as Humans who have progressed to the highest plane of Spirituality, no longer required to return to the Earth. The Middleworld is the world in which we live.Here the god/desses of nature coexist with humans. The Underworld is the realm of shadows, stagnation and rebirth. Souls of the dead go to the Great Cauldron there and await rebirth. It contains the Underworld deities as well as the deities of rebirth, and death. The term Otherworld is used when referring to the Upperworld and the Underworld. All three worlds tend to overlap with the others.

Celtic Shamanic Terminology:

Abred [ir]: The Middleworld. The Plains. Earth.

Annwyn [ir]: The Underworld.

Axis Mundi: The Tree of Life. The Great Tree-Axis.

bith [ir]: the world, or the manifest physical universe.

blath [ir], prosperity: the eastern realm among the Fifths.

cath [ir], conflict: the northern realm among the Fifths.

Ceugant: The Upperworld. The Realm of God (Dannan).

Coiced [ir], a Fifth: A province of the land or cosmos. There are five. They are blath, cath, fis, rige, and seis.

fis [ir], learning: the western realm among the Fifths.

Gwynvyd [ir]: The Upperworld.

magh [ir]: a plane or field used in reference to the higher planes or Upperworlds.

Other-World: a name for the realms in Faerie terminology.

rige [ir], sovereignty: the middle realm among the fifths.

seis [ir], harmony: the southern realm among the fifths.

sidhe [ir]: the realm of the dead, the faerie realm, or the underworld in Faerie Lore.

Tara [ir]: The Middle World.

tir [ir]: this word generally means "land," but also is used symbolically for names of the Underworld.

Tir Na Nog [ir]: The Other-World.

Underworld: a name for the "lower" realms in Celtic Shamanism. These are most often with the Irish word tir (land) in them.
Upper-World: a name for the higher, or celestial realms
in Celtic Shamanism. These are most often with the
Irish word magh (plane) in them.